1. Field of the Invention
The invention relates to armor. More specifically, the invention relates to fiber reinforced composite armor panels.
2. Background
In recent years, forced entry-resistant materials formed from high tensile strength fibers such as aramid fabrics or polyethylene fabrics combined with structural backing have gone into common use. These forced entry resistant materials typically have the advantages of greater tensile strength and the less weight per unit area then metals. Applications for forced entry resistant materials are found in commercial airlines and maritime vessels especially to separate the crew from the passengers.
High-tensile strength fibers such as, for example, aramid fibers in fabrics have been combined with polymer matrices to form polymer-polymer composite armor. These fiber reinforced polymer matrices benefit from the high-tensile strength of the aramid fabric and high resistance to fracture and fatigue of the polymer matrix. Multiple layers of high tensile strength aramid fabric can be combined with epoxy matrices, and compacted into an armor shield.
Composite armor panels have been designed for use in aircraft, vessels, vehicles and buildings. These composite forced entry attack resistant armor panels have combined polymer-polymer fabric layers with structural members to enhance the capacity of the armor for absorbing and defeating attack from multiple threats. These threats include ballistic, forced entry and explosive blast.
Ballistic threats include but are not limited to handguns of various manufactures and calibers. The National Institute of Justice (NIJ) has established a uniform system of calibrating handgun threats, and the ability of armor to defeat these threats. The highest NIJ standard for handgun threats is Level III-A. The NIJ standard for standard rifle threats is Level III. Some handgun configurations have unusually high velocities and ballistic ability that are closer to rifle abilities. These threats have been labeled with the non-NIJ hybrid designation level III-A+. To meet a specific standard, an armor panel must be able to withstand, without failing, five rounds of a particular caliber within one square foot of armor panel. A level III-A capable armor defeats the following calibers and projectile configurations as an example: 25 caliber Automatic Colt Pistol (.25 ACP), .380 ACP, .38 Special, .45 ACP, 9 millimeter (9 mm) Parabellum, .357 Magnum and .44 Magnum. A level III-A+ capable armor defeats the following calibers and projectile configurations as an example: .50 Action Express, flechette (little steel darts) and a one ounce shotgun slug from a 12 gauge shotgun with a 3 inch chambering.
Forced entry attacks include but are not limited to sustained attacks with hand hatchets, fire axes, hand hammers, sledge hammers, chisels, pipes and bats. These represent the typical blunt and sharp attack implements. The American Society for Testing and Materials (ASTM) has adopted a set of standards for calibrating resistance to forced entry attacks. The standard is called ASTM 1233-93. ASTM 1233-93 sets forth procedures whose purpose is limited to the evaluation of the resistance of security systems against the following threats: ballistic impact, blunt tool impacts, sharp tool impacts, thermal stress and chemical deterioration. Failure is denoted by an opening formed in the armor as a result of the above mentioned attacks sufficiently large to get a hand or an object through, or to allow a whole body transfer. ASTM 1233-93 is broken down into multiple classes and each class is divided into multiple sequences. For example class III contains 16 sequences. Class III begins with sequence 1. A test subject that survives sequence 1 goes on to face sequence 2. This progression continues until all of the sequences in the class have been passed or the subject has failed. An armor panel that can withstand a class III sequence 16 standard represents a varied attack of the above threats for about 30 minutes.
Explosive blast as applied to these armor panels includes overpressure and fragmentation or shrapnel effects. A typical test for explosive blast comprises detonating a United States M-67 anti personnel fragmentation grenade three feet from the armor panel. The M-67 grenade weights about 395 grams has a steel casing and contains about 185 grams of explosive. A detonation of a M-67 grenade at this range will generate an overpressure of about 1 to 2 pounds per square inch plus fragmentation effects. Alternatives to the M-67 grenade include but are not limited to the British L2A2 and the German DM51. The L2A2 weights about 395 grams and contains about 170 grams of explosive. The DM51 weights about 435 grams and contains about 60 grams of explosive. Both the L2A2 and the DM51 have similar explosive characteristics to the M-67.
Another requirement for composite armor panels or doors in aircraft is the heat release standard established by Ohio State University (OSU). The maximum heat release rates and smoke density values required by the Federal Aviation Administration (FAA) are described as xe2x80x9cOSU 65/65 200.xe2x80x9d OSU 65/65 denotes a maximum peak and two-minute integrated total heat release for a material which designers wish to place in a commercial aircraft. The maximum peak heat release value is 65 kilowatt per square meter of object (kW/m2). This means that at any one time the most heat that can be generated by the object is 65 kW/ m2. The standard also has a two minute integrated value of 65 kilowatt-minutes per square meter (kW-m/m2). Appended to the OSU standard is a National Bureau of Standards (NBS) standard for smoke density generation with a specific optical density (Ds) maximum of 200. This standard measures the reduction in visibility caused by smoke generated from an onboard fire.
Federal Aviation Regulation 25.853 regulates the combustibility of a material exposed directly to a flame. A Bunsen burner is substituted for an open flame. The vertical Bunsen burner test method is used for determining the resistance of cabin and cargo compartment materials when tested according to the 12-second or 60-second specified in FAR 25.853. Materials subjected to this test pass or fail based on burn length, after flame time, and drip flame time